Author Topic: fiddling with a Poldo tackle  (Read 17642 times)

knotsaver

  • Sr. Member
  • *****
  • Posts: 281
Re: fiddling with a Poldo tackle
« Reply #45 on: June 14, 2015, 10:04:08 PM »
And that's something I'd like to hear Knotsaver explain,
for his using it.  After all, that was part of the OP, that
this structure had proven useful to him (or that he so
believes !).  And I surmise that his use of it is in
the worse condition --rope-through-rope-sheaves.

Yes, you were right in your surmise: rope-through-rope-sheaves!
I've never used the Poldo tackle to lift a load (except last week!? :) ), but I've used it as a boom vang and as a cunningham (see attached picture) on small sailing boats: it often has to be considered an "emergency" tool and/or a cheap one, but it works fine. I've often used it to tighten a rope (for instance to hang the washing, ...). In all of these uses, I don't care about any effort, I tighten as I can or as I need (usually using both hands: acting on the best points (where xarax's purple F forces act), if accessible).
« Last Edit: June 14, 2015, 10:52:38 PM by knotsaver »

xarax

  • Sr. Member
  • *****
  • Posts: 2781
Re: fiddling with a Poldo tackle
« Reply #46 on: June 15, 2015, 12:35:58 AM »
   Somehow you manage to read poorly : his tackle took considerably more force to raise the weight than most other systems --only the 1:1 re-direction was worse.

   I have to tell you that you have not understood what a mechanical system in equilibrium is - because, evidently, you forgot what you had learned back in your high school years  :), and because you try to believe you do understand, even when you do not... Why ? Would nt it much better, to just tie and try knots ? Why do you have to suffer, playing this role, sometimes so poorly as now ?
   You mix and confuse the "ideal" and the "real" system, and the "kinematic" and the" dynamic" calculations of the MA - in short, almost anything you could  ! In an "ideal" system the work done by the force is not "consumed" to raise the weight, for Newton s sake !  The system neither "consumes" nor "generates" energy, as knotsaver told you right from the first sentence of his description.
   Those two "counterbalancing forces" are required only to stabilize the system, NOT to move the weight ! Because it was not in mechanical equilibrium - so, if it was left "untouched", then it would had expanded in no time with an accelerating rate ( as does the Universe ... :)), and it would had reached its maximum extension very soon. Now, when you add those forces, and you have the (vector ) sum of all forces equal to zero ( the forces induced by the weight, the anchor from where the tackle is hanged, and the "counterbalancing" forces required for equilibrium ), then the weight can already be considered as moving, with a constant velocity. Uniform motion is relative, and it does not need ANY force, as Galileo tried to teach you some time ago - in vein, evidently..  If you add more forces in such a system, on top of which are needed to stabilize it, you will destroy this stability, and you will put the whole, or a part of it, in accelerating motion.
  Why, to move a weight in a stabilized "real", now, system, you have to add more forces ? Because you have to overcome friction - and, when you do that, you consume energy, which is transformed into heat. In a "real' system, even if you add more forces, you may not be able to set it in motion with accelerating, or even with constant, speed. Above a threshold, you may overcome static friction, and you may start displacing the weight  - but the forces you will forced to add to achieve this, have no relation whatsoever with the "counterbalancing forces" which you needed only to stabilize the system in the first place.
   Again : In the "ideal", not stabilized Poldo tackle, you do not need to add any force : it will start moving by itself, with an accelerating pace, right from the moment it comes into existence !  :)  In the "ideal", stabilized Poldo tackle, the "counterbalancing forces" are needed only to put it in mechanical equilibrium. Such a system "moves" with constant speed, which for some / relatively to some observers it may be different than it is for some / relatively to some others, or even zero.  THE SYSTEM DOES NOT "MOVE" BECAUSE OF THOSE FORCES ( therefore the weight is NOT lifted because of those forces ) - it "moves", in the way it moves ( with constant speed ) BECAUSE THE SUM OF ALL FORCES IS ZERO !
   I believe that you should tell a student in engineering to explain it to you - you may also ask it from Tex, who seems to be interested in explaining this black hole of knot tyer s Universe : Mechanical advantage.

And it very simply has no " 'higher' pulley " [sic] but is oriented for lateral/horizontal force.

You are suffering:) :)
...and you can not even rotate a picture 90 degrees.
   I was denoting the one pulley as "higher' and the other as "lower", just to facilitate the reading... There is no "weight", no "preferred" orientation of the Poldo tackle, or of any other taclkle ! If you can not rotate the picture, or your head, rotate your computer s screen.

Not only that, butit has no hint of there being an "anchorage" side vs. a "weight/load" side; no, it appears as though it might work on both sides (not "higher"/"lower" things) moving, which is another can of worms to address.

Back to the high school ! ( it would be soo gooood, wound it ?  :) :) )
My God, you do not understand a thing:) :) :)  :) :)  However, I admit that, till now, you had persuaded me that you did more than that, i.e., that you understood more than zero ! Mea Culpa.
Of course which is "anchorage" and which is "weight ' does not matter ! Action and reaction, remember ?  :) Both forces induced into the system are equal, by definition. ( And they are applied on the axles of the pulleys, as I show )
Of course it "might", and it CAN, work "on both sides" ! And of course it can work with any kind of forces applied on its two ends, and in any orientation. Who ELSE in this World, older than 12, can not understand this ?  Back to high school - or to a nice summer place...

 
  *I* have tried to present the practical aspects of the PT..

   That is good, and valuable - you should also tie it on Dyneema, and use free-rotating pulleys, because those are "practical" things, too !  :) . However, the "theoretical" aspects are also valuable - and they may lead to some concrete thing, like the retraced Poldo tackle I had shown, but you had not understood, or you had snubbe, as you do too often ( almost always...). It is very "practical" to have a self-locking tackle / adjustable binder, tied on a sling, with-the-bight ! No ends, no loops, just a multi-folded closed loop, following an endless x=zig zag path. I would nt had thought of it, if I had not tried to present the "theoretical" aspects of the PT ( yet very simple and easy to understand - for somebody who truly wishes to understand, because he does not believe he knows everything, and he is not interested in playing difficult roles in front of a non-existing audience ).
« Last Edit: June 15, 2015, 10:12:36 AM by xarax »
This is not a knot.

xarax

  • Sr. Member
  • *****
  • Posts: 2781
Re: fiddling with a Poldo tackle
« Reply #47 on: June 15, 2015, 01:08:26 AM »
   I believe that the "practical" PT "works" because its zig-zag shape is entangled to aether ( the well-known fifth element ), and so it transforms gravity into non-local anti-gravity.
  (  See the attached "decorative", "colourful" pictures - if you think that they are the same, think again !  :)  )
« Last Edit: June 15, 2015, 09:25:50 AM by xarax »
This is not a knot.

xarax

  • Sr. Member
  • *****
  • Posts: 2781
Re: fiddling with a Poldo tackle
« Reply #48 on: June 15, 2015, 01:14:46 AM »
I've used it as a boom vang and as a cunningham
  Ingenious ! Congratulations !
  I believe you should publish this idea somewhere - I doubt that any other sailor in the world had thought of it.
  The tackles of the booms in the sailing boats are "open" - I have not seen this thing anywhere.
  Perhaps you could also try the retraced Poldo Tacle, in its TIB vatiation where it is tiable with-a-bight. Use a Dyneema sling.
This is not a knot.

knotsaver

  • Sr. Member
  • *****
  • Posts: 281
Re: Retraced Poldo tackle
« Reply #49 on: June 15, 2015, 08:14:02 AM »
  In this double-line / retraced Poldo tackle, we can push or pull the two mid-tackle vertices of the endless zig zag path of the line ( = the tips of the eyes of the loops, in the original tackle ), back and forth - both of them at the same time, or any one of them each time. It is amusing to watch how the tackle expands or contracts - because it is difficult to see how each individual line moves when it slides through the double-line tip af a bight, so all we can do is just to pull the strings, and enjoy the overall spectacle :)

very very nice, xarax!
thanks! :)
are you becoming a Poldo tackle fan too? ;)

 
I have to say that it is not raining, so the umbrella is closed, and it is hanged by the lower U-turn of the tackle. However, I could well had turned it upside down - or hang the tackle itself from the handle of the umbrella, when it will start raining.  :) )
:D
I see, you and Dan are very good friends :)

I've used it as a boom vang and as a cunningham
  Ingenious ! Congratulations !
  I believe you should publish this idea somewhere - I doubt that any other sailor in the world had thought of it.
  The tackles of the booms in the sailing boats are "open" - I have not seen this thing anywhere.

This use is described in the (already cited) book : "I nodi che servono" and I think Poldo Izzo used it in that way too.
(I'm a fan of that short italian knot-book too)
ciao,
s.


xarax

  • Sr. Member
  • *****
  • Posts: 2781
Re: fiddling with a Poldo tackle
« Reply #50 on: June 15, 2015, 09:24:01 AM »
   I mean, I have not seen it used in a sailing boat, in place of a "normal" boomvang or Cunningham.
   I was serious, you should publish it in a sailing magazine !
This is not a knot.

xarax

  • Sr. Member
  • *****
  • Posts: 2781
Re: fiddling with a Poldo tackle
« Reply #51 on: June 15, 2015, 10:06:20 AM »
   When you need it to use as a self-locking adjustable binder, you want to increase friction in the tips of the four bights, not reduce it... So, you should not use pulleys  :), but double nipping loops : just fold the tips of the bight/eyess of the two mid-tackle loops back, and twist them 180 degrees, and then pass the line between the double nipping loops you had formed there, which nips/grips the penetrating lines it even more ! Of course, you would nt be able to adjust this thing under tension - but, once adjusted, it will be locked in this place even more securely than the original Poldo tackle.
« Last Edit: June 15, 2015, 10:13:56 AM by xarax »
This is not a knot.

struktor

  • Sr. Member
  • *****
  • Posts: 304
Re: fiddling with a Poldo tackle
« Reply #52 on: June 15, 2015, 08:44:50 PM »
The Principle of Moments:

2 * F* (2*r/3)  =  F * (4*r/3)


(2*r/3) + (4*r/3) = 2*r


Negative feedback


M = 2*F*(r-R*sin(alfa)) - F*(r+R*sin(alfa))

xarax

  • Sr. Member
  • *****
  • Posts: 2781
Re: fiddling with a Poldo tackle
« Reply #53 on: June 15, 2015, 10:07:19 PM »
   Clever idea - but ad hoc, I am afraid.
   So, you study a system where the end-tackle pulleys are not rotating at all, and they are not even pulleys, but sprockets - and the ropes are chains. OK, this system is stable in the stage you show in your second sketch - and if the end-sprockets are attached to the anchor and to the weight through those solid arms, and if they can not revolve freely around their axles ( which connects them to those arms ), they will remain in mechanical equilibrium, just as you show them : there will be a small angle between the arms and the vertical.
   So What ?
   First, even if the anchor and the weight are located at the centre of the end-tackle pulleys ( i.e., if the arm has zero length, ) the tackle can "work", and we have to explain it. Second, the lines need not be vertical - in fact, in your sketch they will not be vertical, because, if the anchor is located in the centre of the "higher" pulley, and the weight is located in the centre of the lower pulley, the line between those two centres will be vertical, so the lines of the rope will be not.
   However, my main concern is the use of "moments", / inertia ? / momentum ?, which, if I understand how you use it, is a notion related to accelerations and decelerations, not to motion with constant velocity, as static systems are. You are shifting the goalposts - again !  :)
This is not a knot.